@ezzal the best way to do this is to turn off "automatically display" in YARPP's display options, then call the code related_posts() in your theme, under where you display the content, and to wrap that in a conditional so that it's only triggered if you're displaying something in the appropriate category. Needless to say, you'll have to do that in PHP. Hope that helps.

Lets say I don't want the category "cars" to display at all in the related ones. As I have it with a special style, it breaks everything in the related section, so I need to exclude that category from the related posts.

You can exclude particular tags and categories from the "The Pool" options in the YARPP options screen. If you don't see those settings around, you can make them appear by going to the "screen options" tab in the top right.